Conventional ultrasonic diagnostic apparatuses have a configuration in which the intensity of a reflection echo signal obtained as a result of ultrasonic wave irradiation onto a subject is converted into a luminance of a corresponding pixel so that the structure of the subject is displayed in the form of a tomographic image. Further, in recent years, there has been an attempt to measure a movement of a subject precisely by analyzing a phase of a reflection echo signal so as to determine an elastic modulus of the subject based on a result of the measurement.
As Conventional Example 1, a method has been proposed in which high-precision tracking is performed by determining an instantaneous position of a subject using both an amplitude and a phase of an output signal obtained as a result of detection of a reflection echo signal so that minute vibrations in a large amplitude displacement motion caused due to pulsations are captured (see, for example, JP10(1998)-005226 A).
Furthermore, as Conventional Example 2, a method and an apparatus that are based on a development of the method according to Conventional Example 1 have been proposed. The method is such that, with respect to a large amplitude displacement motion of each of inner and outer surfaces of a blood vessel wall caused due to heartbeats, precise tracking for determining a motion speed of minute vibrations superimposed on the large amplitude displacement motion is performed so that a local elastic modulus of the blood vessel wall is determined based on a difference in the motion speed. The apparatus performs a display in such a manner that a spatial distribution of an elastic modulus is superimposed on a tomographic image (see, for example, JP2000-229078 A).
However, Conventional Example 2 described above makes no mention of a method for displaying an elastic modulus image and a tomographic image and an operation of the apparatus. According to Conventional Example 2 described above, in order to measure an elastic modulus, it is necessary to determine an amplitude of minute vibrations by performing tracking of a movement of a blood vessel wall caused in one heartbeat interval. That is, an elastic modulus image is changed only once per heartbeat. It follows, therefore, that since one heartbeat takes about one second, an elastic modulus image has a frame rate of about one frame per second. Meanwhile, generally, a tomographic image is displayed at 15 to 30 frames per second. Thus, when a display is performed in such a manner that an elastic modulus image is superimposed simply on a tomographic image, due to a large difference in frame rate, it is unclear to which portion an elastic modulus corresponds, which has been problematic.